In the field of broadcast communications, electrical filters are required to separate a desired signal from energy in other bands. These bandpass filters are similar to bandpass filters in other fields. However, unlike most other electrical bandpass filters, filters for broadcast communication must be capable of handling a relatively high input power. For example, a signal input to a broadcast communications filter might have an average power between 5 and 100 kilowatts (kW). Many electronic filters do not have the capacity for such large signal powers.
For many years, high power electrical bandpass filtering has included the use of waveguide cavity filters. A variety of different waveguide filter types have evolved, each having its particular benefits and drawbacks. One popular class of filter in the industry is based on a pseudo-elliptical filter function. This type of filter function may be achieved in a number of different ways. Some waveguide bandpass filters make use of the “evanescent mode” to provide coupling between the separate resonators of a filter. In an evanescent mode filter, the waveguide is “below cutoff” (i.e., having a cross-sectional dimension small enough that frequencies within a desired passband cannot proceed normally from one end of the cavity to the other). In such a filter, resonances are formed between the inductance of a section of the waveguide, and the capacitance of a resonator, typically in the form of an adjustable length element projecting into the cavity, such as a tuning screw.